Heretofore, in a process for producing an olefin polymer by use of gas phase polymerization reaction in the presence of hydrogen, there has been known a method for controlling hydrogen concentration within a gas phase polymerization reaction vessel, which comprises lowering hydrogen concentration within the gas phase polymerization reaction vessel by taking out a portion of a hydrogen-containing gas within the gas phase polymerization reaction vessel, then adding hydrogen in the gas taken out to an olefin to hydrotreat the gas, and thereafter feeding the hydrotreated gas again to the polymerization reaction vessel. (Refer to, for example, Patent Document 1.)
Furthermore, in a process for producing an olefin polymer in the presence of hydrogen, there has been known a method for controlling molecular weight distribution or the like of the olefin polymer produced by adding a hydrogenation catalyst to a reaction vessel to lower hydrogen concentration. (Refer to, for example, Patent Document 2.)
[Patent Document 1] JP-A-10-204123
[Patent Document 2] JP-A-8-151408